ORGANIC LETTERS
Decorating BODIPY with Three- and Four-Coordinate Boron Groups
2012 Vol. 14, No. 22 5660–5663
Jia-sheng Lu, Soo-Byung Ko, Nicholas R. Walters, and Suning Wang* Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
[email protected] Received September 24, 2012
ABSTRACT
Two new BODIPY derivative molecules decorated by a Lewis acidic BMes2(vinyl) group and a photochromic four-coordinate boryl chromophore, respectively, have been synthesized. Significant mutual influence on photophysical and photochemical properties by the different boroncontaining units has been observed.
BODIPY (boron dipyrromethene) dyes and derivatives are well-known to display interesting photophysical properties and high photostability, enabling their applications in bioimaging, biolabeling, or probes, etc.1,2 Although most BODIPY dyes are brightly fluorescent in solution, they are usually poor emitters in the solid state. This is caused mostly by “self-absorption” due to the very small Stokes’ shift and large overlap of BODIPY’s absorption and emission spectra. Furthermore, because of the flat conjugated core, the BODIPY fluorophores normally (1) (a) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891. (b) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem. 2008, 120, 1202. Angew. Chem., Int. Ed. 2008, 47, 1184. (c) Ziessel, R.; Ulrich, G.; Harriman, A. New J. Chem. 2007, 31, 496. (2) Lu, J.-S.; Fu, H.; Zhang, Y.; Jakubek, Z. J.; Tao, Y.; Wang, S. Angew. Chem., Int. Ed. 2011, 50, 11658. (3) Bonardi, L.; Kanaan, H.; Camerel, F.; Jolinat, P.; Retailleau, P.; Ziessel, R. Adv. Funct. Mater. 2008, 18, 401. (4) Ozdemir, T.; Atilgan, S.; Kutuk, I.; Yildirim, L. T.; Tulek, A.; Bayindir, M.; Akkaya, E. U. Org. Lett. 2009, 11, 2105. (5) (a) Lu, H.; Wang, Q.; Gai, L.; Li, Z.; Deng, Y.; Xiao, X.; Lai, G.; Shen, Z. Chem.;Eur. J. 2012, 18, 7852–7861. (b) Zhang, D.; Wen, Y.; Xiao, Y.; Yu, G.; Liu, Y.; Qian, X. Chem. Commun. 2008, 4777. (6) (a) Huh, J. O.; Do, Y.; Lee, M. H. Organometallics 2008, 27, 1022. (b) Fu, G.; Pan, H; Zhao, Y.; Zhao, C. Org. Biomol. Chem. 2011, 9, 8141. (c) Yin, Z.; Tam, A. Y.; Wong, K. M.; Tao, C.; Li, B.; Poon, C.; Wu, L.; Yam, V. W. Dalton Trans. 2012, 41, 11340. (d) Sun, H.; Dong, X.; Liu, S.; Zhao, Q.; Mou, X.; Yang, H. Y.; Huang, W. J. Phys. Chem. C 2011, 115, 19947. 10.1021/ol302633s r 2012 American Chemical Society Published on Web 11/06/2012
pack tightly in the solid state, resulting in strong intermolecular interactions, leading to significant luminescent quenching.26 Because of the low fluorescence quantum efficiencies in the solid state (either as a pure solid or in a solid matrix), BODIPY dyes are not suitable for use in applications such as emitters for organic light emitting diodes (OLEDs). To address this deficiency, several research teams have used the strategy of introducing bulky substituent groups including BMes2-phenyl or p-BMes2ph-alkynyl6 (Mes = mesityl) or aliphatic chains at the meso and/or 2,6-positions to strengthen steric hindrance, thus inhibiting intermolecular aggregations.36 This strategy is somewhat effective in improving the solid-state emission quantum efficiency of certain BODIPY dyes. Nonetheless, because the fluorescence of BODIPY originates from the BODIPY core, bulky substituents at meso and/or 2,6-positions often have a significant impact on the emission energy, making the BODIPY chromophore more prone to decay from the excited state via nonradiative pathways, such as vibrational and/or rotational relaxation.1a With the aim of tuning the photophysical properties of BODIPY, especially to increase the emission quantum efficiency in the solid state, we designed and synthesized two new molecules B3a and B3b (Scheme 1). In B3a, a
BMes2-vinyl group is attached to the BODIPY core while in B3b a four-coordinate boron group, B(ppy)Mes2-alkynyl, is attached to the BODIPY core. BODIPY functionalization by these two boryl groups has not been reported previously. These two molecules allow us to examine the impact of three- and four-coordinate boryl groups on the photophysical properties of the BODIPY core. Further, because the BMes2-vinyl group is Lewis acidic7 and the B(ppy)Mes2-alkynyl unit is photochromic,8 these two molecules allow us to examine the influence of the BODIPY core on the anion sensing ability of the three-coordinate boron and the photochromic properties of the N,Cchelate boron unit.
Scheme 1. Synthetic Procedures for B3a and B3b
The syntheses of B3a and B3b are accomplished according to Scheme 1. For B3a, the bis-acetylide BODIPY compound was synthesized first. This precursor compound was connected to the BMes2 unit via a hydroboration reaction with BMes2H using well-established literature procedures,9 producing B3a in good yield. For the synthesis of compound B3b, the N,C-chelate precursor compound8a,b B1 was reacted with LDA first,10 followed by the addition of the parent BODIPY compound, producing B3b in 75% yield. B3a and B3b were fully characterized by 1H, 13C, and 11B NMR spectra, HRMS, and elemental analyses. The two distinct types of boron centers in B3a and B3b were observed in the 11B NMR spectra. For B3a, no cis-isomer with respect to the olefin bond was observed. Both compounds are as (7) (a) Wade, C. R.; Broomsgrove, A. E. J.; Aldridge, S.; Gabbaı¨ , F. P. Chem. Rev. 2010, 110, 3958and references cited therein. (b) J€akle, F. Chem. Rev. 2010, 110, 3985. (c) J€akle, F. Coord. Chem. Rev. 2006, 250, 1107. (8) (a) Baik, C.; Murphy, S. K.; Wang, S. Angew. Chem., Int. Ed. 2010, 49, 8224–8227. (b) Amarne, H.; Baik, C.; Murphy, S. K. Chem.; Eur. J. 2010, 16, 4750. (c) Rao, R. L.; Amarne, H.; Zhao, S. B.; McCormick, T. M.; Martic, S.; Sun, Y.; Wang, R. Y.; Wang, S. J. Am. Chem. Soc. 2008, 130, 12898. (d) Baik, C.; Hudson, Z. M.; Amarne, H.; Wang, S. J. Am. Chem. Soc. 2009, 131, 14549. (9) (a) Charlot, M.; Porres, L.; Entwistle, C. D.; Beeby, A.; Marder, T. B.; Blanchard-Desce, M. Phys. Chem. Chem. Phys. 2005, 7, 600. (b) Yuan, Z.; Entwistle, C. D.; Collings, J. C.; Albesa-Jove, D.; Batsanov, A. S.; Howard, J. A. K.; Taylor, N. J.; Kaiser, H. M.; Kaufmann, D. E.; Poon, S.; Wong, W.; Jardin, C; Fathallah, S.; Boucekkine, A.; Halet, J.; Marder, T. B. Chem.;Eur. J. 2006, 12, 2758. (c) Franz, D.; Bolte, M.; Lerner, H.-W.; Wagner, M. Dalton Trans. 2011, 40, 2433. (10) Goze, C.; Ulrich, G.; Ziessel, R. Org. Lett. 2006, 8, 4445. Org. Lett., Vol. 14, No. 22, 2012
stable as the difluoro BODIPY parent molecule toward air and moisture. The crystal structures of both compounds were determined by single-crystal X-ray diffraction analysis.11 The crystals of both compounds are very thin and small and diffract poorly. As a result, the quality of the crystal data are poor. Nonetheless, the key structural features of both compounds were established unambiguously and are shown in Figures 1 and 2, respectively. The boron atom of the BODIPY core in both compounds has the typical BN (average, 1.56 A˚) and BC (average 1.60 A˚) bond lengths, comparable to those of previously reported BODIPY compounds.16 The BCvinyl bonds in B3a are much shorter (average 1.54 A˚) than those of the BODIPY core, and the BC(mesityl) (average 1.59 A˚), which can be attributed to the π-conjugation of the three-coordinated B atom with the olefin carbon atoms. In fact, the CMesBCMes plane is essentially coplanar with the plane of B-vinyl with an average dihedral angle of 9.5°. The BN and BC bond lengths of the N,C-chelate units in B3b are about 0.1 A˚ longer than the those of the BODIPY core, caused by the steric congestion imposed by the bulky mesityls.
Figure 1. Crystal structure of B3a showing intermolecular interactions between mesityls and the BODIPY core (left) and between two neighboring BODIPY units (right). The boron atoms are shown as pink spheres. Key: blue, nitrogen; gray, carbon.
The most important features revealed by the crystal structures are the two distinct intermolecular interaction patterns for the two compounds. As shown in Figure 1, in the crystal lattice of B3a, the BODIPY unit is sandwiched between two mesityl groups with the shortest atomic separation distances being 3.52 and 3.68 A˚, respectively. The shortest separation distance between two neighboring BODIPY units in B3a is 3.89 A˚. In contrast, the BODIPY core units in B3b are all paired up with the average separation distance between two BODIPY planes being ∼3.50 A˚ (Figure 2), an indication of strong π-stacking interactions. The distinct packing patterns by B3a and B3b may be partially attributed to the shape of these two (11) CCDC 901543 (B3a) and 901544 (B3b) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. 5661
Figure 3. Absorption and fluorescence spectra of B3a (left) and B3b (right) in CH2Cl2.
Figure 2. Crystal structure of B3b showing intermolecular interactions between two neighboring BODIPY units. The boron atoms are shown as pink spheres. Key: blue, nitrogen; gray, carbon.
molecules, one being approximately linear and the other being V-shaped, and are expected to have a significant impact on their luminescent properties in the solid state. As shown by Figure 3, although the absorption spectra contain significant contributions from vinylborane or B1 chromophores, B3a and B3b share a similar emission profile and identical emission maximum (λmax = 510 nm), similar to that of the parent difluoride BODIPY molecule, thus attributable to the BODIPY core.12 Furthermore, the emission energy of both compounds was found to be independent of excitation energy. The lack of emission from the B1 unit8 (λmax= ∼500 nm) in B3b can be attributed efficient internal energy transfer from the B1 unit to the BODIPY core due to the extensive overlap of the absorption peak of the BODIPY core and the fluorescent peak of the B1 unit. TD-DFT computational data also confirmed that the lowest excited states of B3a and B3b with significant oscillator strengths are indeed dominated by electronic transitions of the BODIPY core (Supporting Information). Both compounds have very impressive emission quantum efficiencies (∼100%, see the Supporting Information), much higher than the previously reported BMes2-functionalized BODIPY at the meso or 2,6-positions6 and display little dependence on concentration in solution. However, they do display distinct doping concentration-dependent luminescence in the solid-state blends with PMMA (poly(methyl methacrylate) as shown in Figure 4. At the 5 wt % doping level, the emission spectrum of B3a resembles closely the solution spectrum with λmax = 512 nm and an impressive quantum efficiency of 0.32, which is much higher than the previously reported highest quantum yield (25%) for BODIPY derivatives in PMMA.5a (12) Shah, M.; Thangaraj, K.; Soong, M. L.; Wolford, L.; Boyer, J. H.; Politzer, I. R.; Pavlopoulos, T. G. Heteroatom. Chem. 1990, 1, 389. 5662
At 25 and 50 wt % doping levels, the λmax shifts to 520 and 531 nm, respectively, coupled with a substantial broadening of the spectrum and a decrease of the quantum efficiency (0.30 and 0.18, respectively). At doping levels above 75%, the λmax is further red-shifted to ∼540550 nm, and the emission peak is dominated by excimer emission with a drastic decrease of quantum efficiency (see the Supporting Information). The neat B3a film has a broad emission band with λmax at ∼560 nm and a quantum efficiency of 0.05, albeit weak, a significant enhancement, compared to the parent difluoro BODIPY, which is nonemissive at all either as neat films or powders. The doping concentration-dependent fluorescence in PMMA displayed by B3a may be attributed to molecular aggregations and intermolecular interactions as revealed by the crystal structure shown in Figure 1. The behavior of B3b is quite different. At 5 wt % doping concentration, the PMMA film of B3b appears green but with an emission intensity and a quantum efficiency too low to be determined by our integration sphere. As the doping concentration increases to 25 wt %, the emission color of B3b becomes red with λmax at ∼630 nm, which is about 120 nm red-shifted, compared to the solution fluorescence spectrum. This red shift is much more dramatic than that observed in B3a. Most importantly, further increasing of the doping concentration does not cause further change the emission spectrum. The highest quantum efficiency of B3b in PMMA films was observed to be 0.10 at the 50 wt % doping level, much lower than that of B3aPMMA films at the same doping level. The peculiar concentration-dependent behavior of B3b in PMMA films can be explained by the “stacked dimer” formation, shown in Figure 2, which becomes dominant and persistent at doping levels greater than 25 wt %. The red emission band displayed by B3b can therefore be assigned to excimer emission of the π-stacked BODIPY dimer.13 The distinct concentration-dependent behavior of B3a and B3b in the solid state illustrates that the fluoride substitution in (13) (a) Saki, N.; Dinc, T.; Akkaya, U. Tetrahedron 2006, 62, 2721. (b) Mikhalyov, I.; Gretskaya, N.; Bergstroem, F.; Johansson, L. B. A. Phys. Chem. Chem. Phys. 2002, 4, 5663. (c) Marushchak, D.; Kalinin, S.; Mikhalyov, I.; Gretskaya, N.; Johansson, L. B. A. Spectrochim. Acta, Part A 2006, 65A, 113–122. (d) Bergstroem, F.; Mikhalyov, I.; Haeggloef, P.; Wortmann, R.; Ny, T.; Johansson, L. B. A. J. Am. Chem. Soc. 2002, 124, 196. Org. Lett., Vol. 14, No. 22, 2012
Figure 4. Fluorescence spectra of B3a (top) and B3b (bottom) doped PMMA films and neat films (100%) (the fluorescence spectrum of the 5 wt % of B3b-doped film could not be obtained due to the very weak signal). Inset: photographs showing the fluorescence colors of the B3a- or B3b-doped PMMA or neat films.
BODIPY by a BMes2-vinyl and a B(ppy)Mes2-alkynyl can be used as an effective strategy to control/influence the solid state luminescence properties of BODIPY. The impact of the boryl substituents on the electronic properties of the BODIPY can be further evidenced by the reduction potentials of B3a and B3b. The first reduction peak of both compounds is characteristic of BODIPY reduction. However, the first reduction potential of B3a (1.95 V, relative to FeCp20/þ) is much more negative than that of B3b (1.72 V), indicating that despite the lack of direct π-conjugation between the boryl and the BODIPY core the boryl substituents do have a significant impact on the BODIPY unit. This may be explained by the much greater electron affinity of the B1 unit that is known to have a much more positive reduction potential8a,b than the BMes2(vinyl) unit14 does. In agreement with the electrochemical data, DFT computational results show that the LUMO of both compounds is localized on the BODIPY unit with the energy level of B3b being about 0.30 eV lower than that of B3a (Supporting Information). (14) Entwistle, C. D. Conjugated organic and organoboron materials, Durham Theses, Durham University, 2005. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/3932/. Org. Lett., Vol. 14, No. 22, 2012
To examine the impact of the BODIPY unit on the Lewic acidity of the BMes2-vinyl group in B3a and the photochromic properties of of the B1 unit in B3b, we carried out fluoride titration for B3a and photoisomerization experiments for B3b, respectively. The addition of TBAF to the THF solution of B3a resulted in a partial quenching of the BODIPY absorption and emission peak in the UVvis and fluorescence spectra, respectively, with binding strength similar to those observed previously for typical BMes2(vinyl) compounds.15 This observation supports that although fluorescence of B3a is dominated by the BODIPY unit, it is electronically coupled to the BMes2(vinyl) unit. TD-DFT also supports that the singlet excited states (S1 and S2, which are very close in energy) responsible for the observed fluorescence do have appreciable contributions from the charge transfer transition of BMes2-vinyl to BODIPY (see ESI). In the photoisomerization experiments, compound B3b was irradiated in toluene or C6D6 at 365 nm (the chargetransfer band responsible for photoisomerization of B1) and the solution was monitored by UVvis and 1H NMR spectra. After extended irradiation, no change was observed for the solution of B3b by either UVvis or NMR spectra. This lack of any photochromic activity by the B1 unit can be attributed to a fast and efficient intramolecular energy transfer from the B1 unit to the BODIPY core since the optical energy gap of B1 (∼3.0 eV) is much greater than that of BODIPY (∼2.3 eV) (Figure 3), which completely intercepts the excitation energy, thus blocking photoisomerization of the B1 unit. In summary, two new BODIPY molecules decorated by a BMes2(vinyl) unit and an N,C-chelate boryl unit, respectively, have been achieved. Unlike substitution at the meso or the 2,6-positions of the BODIPY core that significantly shifts the emission energy, the difluoro substitution by a BMes2-vinyl or a B(ppy)Mes2-alkynyl group does not shift the emission energy of the molecule in solution, compared to the difluoro BODIPY parent molecule. Both molecules have ∼100% emission quantum efficiencies in solution and display distinct doping concentration-dependent fluorescence in a PMMA matrix. The BMes2(vinyl) unit greatly enhances BODIPY fluorescence efficiency in a PMMA matrix while the photoreactivity of the N,C-chelate boryl unit is switched off completely by BODIPY via intramolecular energy transfer. Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada for financial support. Supporting Information Available. Synthetic and characterization details, crystal structural data, solid-state luminescent quantum efficiencies at various doping levels, TD-DFT data, and CV and TBAF titration data. This material is available free of charge via the Internet at http://pubs.acs.org. (15) Pammer, F.; J€akle, J. Chem. Sci. 2012, 3, 2598. The authors declare no competing financial interest. 5663
